1. Field of the Invention
The present invention pertains to sonar sensors. More particularly, the invention relates to a system for sensing the direction of travel of a water borne, flexible, towed array of acoustic transducers or hydrophone sensors being drawn by a towed body which in turn is being pulled by a ship.
2. Description of the Prior Art
It is well known in the field of sonar systems to pull a submersible towed body behind a water craft. The towed body in turn draws behind it one or more flexible towed arrays of water borne hydrophone sensors. Such sensors are useful for detecting the position of underwater sonar contacts. Towed arrays of hydrophones are well known in the art. Such are long, continuous, neutrally buoyant hose lines, often several hundred meters long, which periodically contain numerous hydrophone receivers. Typically, these elongate arrays have a number of sensor elements in modules, along with their associated electronics and control leads, contained within a hose-shaped structure that extends through the water. The hydrophones are connected by suitable electronics on a towing vessel which indicates the bearing of an identified underwater target. Exemplary towed arrays are described in U.S. Pat. Nos. 4,554,650 and 5,412,621. In locating submerged bodies, sonar is either used passively, through reception of signals sent through water, or actively. In an active sonar system a pulse of sound is sent into the water by a sonar projector. The sound is then reflected back from the target and detected by the hydrophone arrays as an echo. This echo is received as mechanical energy by an acoustic transducer, converted into electrical energy, and read by a computer on an attached vessel to pinpoint the location of objects within the water. As shown in U.S. Pat. No. 5,844,860, elongated, hose-like towed arrays of hydrophones attached to the rear end of towed bodies are commonly used for the acoustic sensing of moving objects within the ocean.
A problem with the typical towed array is that it must be properly aligned or the acoustic signals received may not indicate the true horizontal direction of underwater objects. To accurately locate the position of an underwater object it is necessary to know the direction of the towed arrays with respect to the direction of the towed body axis. The towed body tends to react to the movement of the ship as it is pulled through water, while the towed arrays line up with the varying water current flows. Frequently, a yaw angle between the axis of a towed body and the direction of water current flow across the body will form during travel through water. The angular difference between the towed body axis and water flow angle or "yaw" angle will cause a contact bearing error in sonar systems which have free streaming towed array sensors attached to a towed body. Thus a yaw angle in the towed body with respect to current flow will cause errors for target bearing and skew the arrays relative to each other. Designs of existing towed arrays systems use one or more small and expensive heading sensors in each array to sense array direction. One solution to sensing the direction of the towed arrays is to incorporate magnetic compass sensors within the towed arrays, but these are very expensive due to their small size and the requirement to operate with the towed arrays rotated to any angle about their towing axis. These small heading sensors are expensive, difficult to integrate into the array and require elaborate and expensive facilities to perform calibration once installed. Such small heading sensors are about one inch in diameter and seven inches long. One cannot spin an array around its horizontal axis to calibrate the heading sensor in a fixed magnetic field so a calibrated magnetic field created by a set of computer driven coils arranged in a large cube is spun around the sensor within the laid out array (Helmholtz coil arrangement). Also, the sensor must be able to sense the magnetic field in any rotational orientation about its axis so the design has a 360 degree tilt sensor which is used to compensate the heading measurement as the array rolls relative to the horizontal.
A more effective alternative method for sensing and correcting a yaw angle would be desired. A lower cost solution is to provide a single low cost, self calibrating heading sensor in the towed body along with the towed body yaw sensor. The towed body heading sensor measures the bearing of the towed body centerline which is ideally aligned with the water flow. The yaw angle sensor measures the angle of the towed body relative to the water flow direction.
The present invention provides a towed body yaw angle sensor arrangement where a vessel draws a towed body and the towed body pulls a towed array. A submersible towed body is provided with a pivoting fin extending outwardly from a surface of the body, which fin aligns with the direction of flow of water along the fin. A magnetic compass is fixed to the body which calibrated to the central axis of the body and determines a heading of the central axis of the body. Suitable means determine an angle between the fin and the central axis. Since the fin and the towed array are free to align with the direction of water flow, the fin direction is indicative of the array direction. Thus the angle between the fin direction and the axis of the towed body indicates a correction angle between the array direction and the towed body heading. This angle corrects the bearing angle of a target as reported by the array.